Synchronous generator having auxiliary power windings and variable frequency power source and method for use

ABSTRACT

A synchronous generator is disclosed having main power windings and auxiliary power windings, where the auxiliary power winding is coupled to a variable frequency drive system. The variable frequency drive causes the generator to function as a motor and turn a drive shaft to start a gas turbine. Switching circuits are used to connect and disconnect the auxiliary windings of the generator with the variable frequency power supply.

This application is a division of application Ser. No. 09/571,812, filedMay 16, 2000, now pending, the entire content of which is herebyincorporated by reference in this application.

BACKGROUND OF THE INVENTION

The invention generally relates to the field of synchronous powergenerators, such as those used in combination with gas turbines.Specifically, the invention relates to synchronous generators havingboth a power and auxiliary windings.

Synchronous power generators are commonly used by power utilities toproduce electrical energy. Generators generally have a magnetic rotorthat is surrounded by a stationary stator having conductive windings.Rotating magnetic field from the spinning rotor creates electric currentin the armature windings in a stationary stator that surrounds therotor. The current from these windings is output as electrical powerfrom the generator. The stator generally has two or three armaturewindings, each of which have an induced current. These currents aresynchronous, but out-of-phase with each other. The generator producestwo- or three-phase alternating current as electrical power usable byelectric power utilitliy companies.

Synchronous power generators are often driven by gas turbines. Gasturbines have a rotating drive shaft that is coupled to the drive shaftand rotor of the generator. When running, the gas turbine turns thedrive shaft and rotor causes the power generator to produce electricity.In these gas turbine and generator power units, the generator iscommonly adapted to alternatively function as a starting motor for thegas turbine. To start the gas turbine, the generator may be temporarilyoperated as a motor that is powered from an auxilary electrical powersource. Once the generator/motor accelerates the rotational speed of thedrive shaft sufficiently to start the gas turbine, the gas turbine isstarted. Once started, the gas turbine begins to output power to thedriving the drive shaft and the generator, and the motor is switchedback to operate as a generator.

A variable frequency power supply that drives a generator as a motor tostart a gas tubine is referred to as a “static start” drive. The staticstart variable frequency power supply applies current to the statorwindings of the generator. The magnetic fields created by the current inthe stator windings cause the generator rotor to turn which, in turn,powers the drive shaft. The power supply gradually increases thefrequency of the current applied to the stator to increase therotational speed of the rotor. As the rotational speed of the rotor anddrive shaft increases, the turbine is accelerated to its rated startingspeed, and the turbine becomes self-sustaining and generates outputpower to drive the generator.

The General Electric Company has previously sold and marketed gasturbine and generator power units that have “static start” capabilities.FIG. 1 illustrates a conventional three-phase synchronous generator 10that is coupled to a static start drive 12 which provides variablefrequency power to drive the generator 10 as a motor in order to start agas turbine. The static start drive 12 is switchably coupled to thethree-phase output lines 14 of the armature of the generator. The outputof the generator is normally connected to a closely balanced powertransmission system 16. A disconnect breaker or other switch 18 connectsthe static start drive 12 to the output line 14 of the generator. Whenthe breaker 18 is closed, the static start drive 12 applies power todrive the generator as a motor and start the gas turbine.

Power to drive the static start drive 12 is provided by an auxiliarypower bus 20. The static start drive provides a variable frequency powerto drive the generator as a motor during the gas turbine start-up mode.Once the gas turbine is running and self-sufficient, a disconnectbreaker or switch 18 disconnects the static start drive from the outputpower lines 14 of the generator. The generator ceases being a motor andinstead becomes a generator driven by the gas turbine. Electrical powerprovided by the generator can be applied to the balance of the powersystem which requires electrical power from the generator.

The static start drive is typically formed of non-moving (hence the termstatic) solid-state devices such as a load-commutated inverter (LCI) orpulse-width modulated (PWM) drive formed from solid-state rectifiers,diodes and other such devices. The LCI or PWM may be used to provide avariable frequency power supply from the constant frequency power supply(typically 50 Hertz (Hz) or 60 Hz) provided from the station auxiliarypower bus 20. The excitation supply 22 is also powered by the auxiliarypower bus 20 and is coupled to a field winding 24 of the generator.

The static start drive system generally uses a variable frequency powersupply having sufficient capacity to be compatible with the generatorarmature winding terminals. For existing gas turbine generator systems,the armature voltage is typically in the range of 10 kV to 20 kV. Thevoltage of the static start drive is typically in the range of 2.3 kV to7 kV. This range is sufficiently close to the normal operating range ofthe generator armature voltage for the static start drive to be applieddirectly to the main power windings of the generator. Moreover, for aproperly-designed static start system, the voltage and currentspecifications of the static start drive matches the electricalcharacteristics of the generator to provide the required acceleratingtorque to the generator rotor. This matching of the static start drivesystem to the armature voltage of the generator is possible forgenerators having normal operating ranges of 10 kV to 20 kV.

FIG. 2 shows a conventional generator having an auxiliary power windingused for exciting the generator when the generator is operating ingenerator mode. In particular, a generator 10 having a three-phase mainpower winding output 26 that provides power to a balance of the powersystem 16, including the power system beyond the generator terminals,such as transformers, circuit breakers, transmission lines and otherelectrical loads in the power system. In addition, the generator hasauxiliary power windings which are represented by the outputs lines 28to those windings. The General Electric Company has developedsynchronous generators having both main power windings and auxiliarypower windings. In particular, the GENERREX™ excitation system includesauxiliary power windings in synchronous generators, such as is describedin U.S. Pat. Nos. 4,910,421; 4,682,068 and 4,477,767.

In addition, auxiliary windings in synchronous generator/motor machineshave been proposed in Naval ship propulsion systems. In particular, themain generator windings would provide power to electric motors coupledto the propeller shaft. The auxiliary windings would provide power tothe shipboard power distribution system for lights, motors and othership functions.

High voltage generators have been developed that operate at normaltransmission line voltages of 40 kV to 400 kV. These generators produceoutput power in the range of normal transmission line voltage, which issubstantially greater than the output voltage range of prior generatorsand well beyond the voltage range of the power supply for a static startsystem. High voltage generators have armature windings that operate in40 kV to 400 kV. It is believed to be impractical to couple a staticstart drive (which operates in the range of 2.3 kV to 7 kV) to supplythe high voltage windings in a high voltage generator while providingreasonable starting torque. Accordingly, there is a need for staticstart drive system which may be coupled to high voltage generators.

In addition, the static start drive system can be applied to synchronousgenerators which are being operated as synchronous condensers.Synchronous condensers are operated in power systems to supply reactivepower to assist in maintaining desired voltage levels. The condensersprovide no real power to the system. Synchronous condensers aregenerally applied to provide reactive power that leads real power tooffset or cancel normal lagging reactive power in a power load system.

BRIEF SUMMARY OF THE INVENTION

A novel system has been developed which combines a static start drivewith auxiliary power windings to provide a generator with the ability touse high voltage generators to start gas turbines, serve as asynchronous condenser, and perform other functions. In one embodiment ofthe invention, a static start drive is connected to the auxiliarywinding of a high voltage synchronous generator. The auxiliary windinghas winding characteristics, e.g., such as voltage and currentcapabilities, that are suitable for connection to a static start drive,or other auxiliary load or excitation system. The main windings of thegenerator may be designed for much higher voltages and would not besuitable for a static start drive.

Applying a static start drive to an auxiliary power winding of a highvoltage generator allows the static start system to be used with a highvoltage generator. Starting a gas turbine using a variable frequencypower supply attached to an auxiliary winding in a high voltagegenerator allows the static start technique to be used with a widervariety of generators, e.g., high voltage generators than was previouslyavailable. In addition, providing two windings in the generatorincreases the flexibility in its uses. By separating the main windingsfrom the auxiliary windings, the characteristic of each type of windingsmay be selected to best suit different applications of the samegenerator. In particular, the auxiliary windings may be designedspecifically to work in conjunction with the static start drive, and themain windings may be designed to connect to a high voltage transmissionline.

Alternatively, the auxiliary windings to the static start drive may havepreviously existed in the generator for other purposes. For example, theGENERREX excitation system employs auxiliary windings that are separatefrom the main power windings and are formed by a series of conductorsplaced in the stator slots with the main windings. These conductors formdistinctive three-phase auxiliary windings separate from the mainwindings. By making use of existing auxiliary windings in a generator, agenerator may be converted to include a static start drive systemwithout making major internal modifications to the generator. Forexample, a static start system may be added to a high voltage generatorwith existing auxiliary without adding additional stator windings andwithout having to couple the static start drive to the main statorwindings of the generator. In addition, use of auxiliary windings in ahigh voltage generator provides a means suitable for placing balancedcurrents in the winding region of the stator core. The combination ofconductor placement and current forms a rotating magnetic field fordriving the generator as a motor.

Balanced windings and balanced currents are used to create a magneticfield that rotates within the motor or generator at synchronous speed.Windings are said to be balanced if they are geometrically identical andequally placed around the periphery of the stator core (every 120°).Currents are said to be balanced if the current in each of the phaseshas the same magnitude (amperes), and the time phase of the alternatingcurrents differ by a predetermined angle, typically 120° for a threephase winding. Balanced currents flowing in the balanced winding producea magnetic field in the gap between the rotor and stator that isconstant in magnitude and spins in synchronism with the rotor.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and benefits of the present invention will bemore fully understood by careful study of the following more detaileddescription of preferred exemplary embodiments of the invention.

FIG. 1 is a schematic diagram showing a conventional static start drivesystem coupled to a generator;

FIG. 2 is a schematic diagram showing a conventional generation systemcoupled to an auxiliary power winding of a generator;

FIG. 3 is a schematic diagram of a novel static start system coupled toan auxiliary power winding of an auxiliary generator, and

FIG. 4 is a schematic diagram of a three-phase main windings systemhaving taps to provide an auxiliary winding function for a generator.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

FIG. 3 shows a schematic diagram that generally shows a generator 50having a stator 51 with both main power windings and auxiliary powerwindings represented by the main power winding armature output 52 andauxiliary power winding armature output 54. In the embodiment shown inFIG. 3, the main power winding armature 52 carries power to a balancedpower load system 56. The load system may be a line transmission voltagesystem typically operating in the range of 40 kV to 400 kV. The voltagesin these ranges may be directly provided from the generator 50 over itsmain power windings 52 to the balanced power system.

In addition, the generator 50 has an auxiliary power winding 54 thatprovides electrical power via its armature system to a balancedauxiliary power system 58. The auxiliary power system 58 may be a lowvoltage system or other auxiliary power load system. The auxiliary powerload system may constitute the electrical loads in the power plantassociated with the generator. A high voltage generator may output powerat 400 kV, for example, which is too high for use within the plant. Theauxiliary winding provides low voltage power from a high voltagegenerator.

The generator is able to provide electrical power via its main powerwindings to a power load system 56 which is operated at differentvoltages and currents than is the auxiliary power load system 58 whichis also powered by the generator. The generator drives both main powersystem and auxiliary power system because it has main power windings 52and auxiliary power windings 54.

A static start drive 60 may be coupled to the auxiliary power winding 54of the generator. The static start drive may be a variable frequencypower supply used to operate the generator as a motor and turn the rotorat a variable rotational speed proportional to the frequency of thestatic start drive. The rotor of the generator turns a drive shaft 61coupled to a gas turbine 63 to start that turbine. The static startdrive may also include a load-commutated inverter (LCI) or pulse widthmodulated (PWM) drive. Components for the variable frequency power drivemay include rectifiers, diodes and other advanced solid-state electroniccomponents. Driving power for the static start drive is provided from astation auxiliary power bus 62, which may provide three-phase current at50 Hz or 60 Hz.

The static start drive is switchably connected to the auxiliary powerwinding 54 through a disconnect breaker or switch 64. Similarly, theauxiliary power bus 62 is switchably connected to the balance of theauxiliary power system 58 by a second disconnect breaker or switch 66.The disconnect breaker switch 64 for the static start drive is closed tocouple the static start drive to the auxiliary power windings of thegenerator 50 when the generator is to be used as a motor to start a gasturbine, for example. The disconnect breaker 64 for the static startdrive is closed to engage the static start drive to the auxiliary powerwindings of the generator. At the same time, the disconnect breaker 66between the auxiliary power winding and balanced auxiliary power system58 is opened to disconnect the auxiliary power winding from theauxiliary power system.

The breakers 64, 66, ensure that while the static start drive 60 isproviding variable frequency power to the auxiliary power winding of thegenerator, the auxiliary power system 58 is not drawing power from thestatic start drive or generator. In addition, during static startoperations, the auxiliary power system 58 should be disconnected fromthe auxiliary power winding 54 to avoid feeding variable frequency powerinto the auxiliary power system 58. Alternatively, when the static startdrive is disconnected from the auxiliary power winding (such as afterthe gas turbine has been started and is self-sustaining), the firstdisconnect breaker 64 is opened and the second disconnect breaker 66 isclosed. Thus, when the static start is off, the balance of the auxiliarypower system 58 receives current from the auxiliary windings andprovides that current to the auxiliary power bus 62.

An excitation supply 68 provides magnetizing power to the rotatinggenerator field winding 70. Typically, the excitation system providesdirect current (d.c.) to the field winding 70 at relatively-low voltages(300 to 700 volts) compared to the stator voltage in the generator 50.During the gas turbine start sequence, the excitation supply 68 suppliespower to the field winding 70 at various levels that are functions ofthe turbine-generator speed. The static drive 60 and the excitationsupply 68 are linked with a control circuit to provide the proper powerlevels during the turbine start sequence. Power for the excitationsupply 68 may be provided from the auxiliary power bus 62.

The main windings of the generator 50 may have armature voltages in therange of 40 kV to 400 kV, which is well above the voltages suitable forstatic start drive systems. The auxiliary winding may have armaturewinding voltages in the range of 2 to 7 kV, which is suitable forconnecting to a static drive system. The winding voltages may increasewith advances in solid state electronics. Accordingly, the presentsystem allows for a static start drive 60 to be coupled to a highvoltage generator 50. The auxiliary windings 54 may be sized to matchthe variable frequency power supply provided by the static start drive60.

FIG. 4 is a schematic circuit showing an auxiliary power winding formedby tapping the turns of a main power winding. A three-phase main powerwinding system 72 is shown by the three branch windings 74, 75 and 76 ofthe main windings of generator 50. The outputs of the main windings 78,80 and 82 provide three-phase power, such as high voltage power fordirect connection to line transmissions. The main windings have powertaps 84 that connect to the end turns of one of the first few turns ofthe main power winding. These taps allow current to be extracted fromthe main windings at a point where the voltage is relatively low ascompared to the voltage across all of the turns in the main windings. Bytapping the first few turns of the main power winding, an effectiveauxiliary winding may be created which provides polyphase (e.g., threephase), low-voltage and low-amperage current at an auxiliary connectionbar 86. The auxiliary connection bar 86 may be used to couple to thefirst breaker or switch 64 and to the balance of the auxiliary powersystem 58.

The present static drive system coupled to auxiliary windings may begenerally applied to polyphase synchronous electrical machines duringtheir start-up phases. As described above, the present system isparticularly suitable for use with generators that operate in connectionwith gas turbines and provide output voltages that are substantiallygreater than those voltages of typical power conversion systems, such asrectifiers, LCI and PWM drives. Moreover, the present system may beemployed with a variety of auxiliary windings, including those that tapmain power windings, are formed of additional conductors included withmain windings in a stator, and other polyphase synchronous electricalmachines that have auxiliary winding systems which match static startdrive systems.

While particular exemplary embodiments of the present invention havebeen described and illustrated, it should be understood that theinvention is not limited to the disclosed exemplary embodiments.Modifications and variations may be made by persons skilled in the artwhile still retaining some or all of the advantages of this invention.The present invention is intended to include any and all suchmodifications within the spirit and scope of the following claims.

What is claimed is:
 1. A method for starting a gas turbine using apolyphase electric power generator comprising the steps of: a.connecting a variable frequency power supply to auxiliary windings ofthe generator; b. applying current to the auxiliary windings of thegenerator to turn a drive shaft of the generator; c. rotating a gasturbine with the turning rotor of the generator to start the gasturbine, and d. once started, the gas turbine turns the generator tocause the generator to produce electric power from the main armaturewindings.
 2. A method for starting a gas turbine using a polyphaseelectric power generator as in claim 1 further comprising the step ofdisconnecting the variable frequency power supply from the auxiliarywindings after the gas turbine is started.
 3. A method for starting agas turbine using a polyphase electric power generator as in claim 1further comprising the step of increasing the frequency of the powerapplied by the variable frequency power supply to accelerate therotational speed of the drive shaft as the gas turbine is being started.4. A method for starting a gas turbine using a polyphase electric powergenerator as in claim 1 wherein the generator is a three-phase generatorhaving three main windings having an output voltage in a range of 40kilovolts (kV) to 400 kV, and three auxiliary windings having an outputvoltage of less than 10 kV.
 5. A method for starting a gas turbine usinga polyphase electric power generator as in claim 1 further comprisingthe step of disconnecting the variable frequency power supply after thegas turbine is started and connecting the auxiliary windings to anauxiliary power system.